Neanderthals were stockier than anatomically modern humans who shared the planet with them at the time of their demise, but their brains were the same size, the team write in the journal Proceedings of the Royal Society B.

As a result, Neanderthals "would have required proportionately more neural matter" to maintain and control their larger bodies, they say.

Larger eye sockets

Comparing the skulls of 32 H. sapiens and 13 Neanderthals, the researchers also established the Neanderthals had significantly larger eye sockets, indicating bigger eyes and visual cortices - those areas of the brain that regulate vision.

"More of the Neanderthal brain would have been dedicated to vision and body control, leaving less brain to deal with other functions like social networking," says Oxford anthropologist and lead author Eiluned Pearce, a doctoral student.

Among living primates and humans, the size of an individual's social groups is constrained by the size of specific brain areas, she says. The larger these areas are, the more connections an individual can maintain.

The archaeological record seems to support the theory that Neanderthals were cognitively limited to smaller groups - they transported raw materials over shorter distances and rare finds of symbolic artefacts suggest a limited ability to trade.

The ability to organise a collective response would have been a key to survival when times turned harsh, like during the ice age, Pearce says.

"If Neanderthals knew fewer people in fewer neighbouring groups, this would have meant fewer sources of help in the event of, for example, local resource failure," she says.

"Smaller groups are also more liable to demographic fluctuations, meaning a greater chance of a particular group dying out. Smaller groups are less able to maintain cultural knowledge, so innovations may be more likely to be lost.

"Overall, if Neanderthals had smaller groups ... this could have led to their extinction along a variety of pathways."

Humans won out

Previous research by the Oxford scientists shows that modern humans living at higher latitudes evolved bigger vision areas in the brain to cope with the low light levels.

This latest study builds on that research, suggesting that Neanderthals probably had larger eyes than contemporary humans because they evolved in Europe, whereas contemporary humans had only recently emerged from lower latitude Africa.

"While the physical response to high-latitude conditions adopted by Neanderthals may have been very effective at first, the social response developed by anatomically modern humans seems to have eventually won out in the face of the climate instability that characterised high-latitude Eurasia at this time," the study concludes.

The relationship between absolute brain size and higher cognitive abilities has long been controversial, the authors admit.

Their finding, that similar-sized brains had been differently organised, "could explain why Neanderthal culture appears less developed than that of early humans, for example in relation to symbolism, ornamentation and art".